Emulsion polymerization technology is a well established means for preparing latex materials. Latex materials can then be finished to form a dried resin that is useful in a variety of applications, such as raw materials for tire and other rubber manufacturing processes or toner for photocopying machines. Processes for finishing latex to form a dried resin generally involve: recovering the polymer from the aqueous suspension; and processing the polymer to get it into the form of a dry resin.
Aqueous emulsion polymerization, typically, involves copolymerizing monomers, such as butadiene and styrene, in a water phase in the presence of a polymerization initiator, emulsifier and modifier. Generally, the copolymerization is performed at a temperature necessary to effect reaction of the styrene and butadiene, for example, and is continued until the desired conversion is attained. A latex results from this polymerization. The latex is coagulated, in typical processes, and the coagulum is washed and dried in order to form a dry resin.
U.S. Pat. No. 5,247,034, which issued on Sep. 21, 1993 to Mate et al., discusses emulsion polymerization technology with respect to producing a toner resin. Mate et al. disclose improving particular characteristics of these toner resins by adding amino acid soaps to the emulsion during processing.
An emulsion polymerization and latex dewatering process is disclosed in U.S. Pat. No. 2,615,206, which issued to Paul Lindstedt on Oct. 28, 1952. Lindstedt discloses syneresis, which as used herein is the contraction of a gel standing with the exudation of liquid. The Lindstedt process includes heating the slurry resulting from coagulation of an emulsion polymerized latex at a temperature that is below the agglomeration or softening temperature of the polymer until a mass of discrete particles is obtained and a form-sustaining mass is formed. This mass of particles (i.e. resin cake) is then shaped, in the Lindstedt process, by forcing the cake through a grid in order to form rods. These rods are then heated in a heating chamber at a temperature above the agglomeration temperature and dried in a heating chamber to form a resin having a moisture of about 0.75% by weight on a bone dry basis.
Generally for dewatering processes, if a substantial portion of the water is evaporated from the latex, then there is a tendency for the concentration of residuals in the resin to be higher than in resins having most of the water exuded as a liquid. This high residual level is most likely due to the fact that residuals can be carried away from the resin in liquid (e.g. water) that is exuded from the resin. However, typically, these residuals do not evaporate with the water.
In general, many processes for finishing latex to form a dry resin have a tendency to breakdown the thermoplastic polymer in such a manner that physical properties of the polymer, such as melt flow index and molecular weight distribution, can vary greatly for the polymer in the latex and the resulting dry resin. These modified characteristics are typically caused by mechanical or thermal breakdown of the polymer, which occurs under particular types of processing conditions subsequent to coagulation of the latex.
Therefore, there is a need for a process to finish latex to form a dry resin that minimizes the residuals concentration in the dried resin and that does not significantly alter polymer characteristics, such as molecular weight distribution and melt flow rheology.